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Effect of reference-ET method on irrigation scheduling model

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  5th National Decennial Irrigation Conference Proceedings, 5-8 December 2010, Phoenix Convention Center, Phoenix, Arizona USA  IRR10-9814.(doi:10.13031/2013.35838)
Authors:   Daniel K Fisher, H.C (Lyle) Pringle III
Keywords:   irrigation scheduling, evapotranspiration, water balance, soil-water deficit

A commonly used method of scheduling irrigations involves maintaining a soil-water balance and estimating a daily soil-water deficit. The water balance accounts for water moving into the soil (as irrigation or rainfall) and out (as crop evapotranspiration or runoff). A critical component, evapotranspiration, is often estimated by calculating a reference evapotranspiration (ETo) and adjusting it with a crop coefficient function. Many ETo models are available which use various types of weather data and range from simple empirical equations to highly sophisticated, physically based models. The various methods produce a range of estimates which can vary considerably, affecting daily crop evapotranspiration and water-balance and soil-water deficit calculations. In the humid Mid-South region, rainfall can be frequent during the growing season, replenishing the soil-water reserves and resetting the deficit in the soil-water balance. A question arose regarding the required accuracy of ETo estimates: under conditions of frequent rainfall, was it necessary to use a sophisticated, data- and calculation-intensive ETo model or could a simpler model be used without greatly affecting the irrigation schedule? This paper discusses an irrigation scheduling simulation for two crops during 13 growing seasons using two scheduling models, a spreadsheet water-balance model and a computer program (the Arkansas Irrigation Scheduler). A variety of ETo methods were used, and the impact of the ETo estimates on daily seasonal irrigation schedules was examined.

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